Computer tomography is an imaging method which is primarily used for medical diagnostics and for examining material. In the context of computer tomography, a radiation source (e.g. an x-ray source) and an associated detector device rotate about an object that is to be examined, in order to record spatially three-dimensional image data. During the rotational movement, measurement data is recorded within an angle sector. The projection measurement data represents a plurality of projections containing information about the attenuation of the radiation by the examination object from various projection angles. These projections allow a two-dimensional sectional image or a three-dimensional volume image of the examination object to be calculated. The projection measurement data is also referred to as raw data and may be preprocessed, e.g. in order to reduce any attenuation intensity differences caused by the detectors. Image data can then be reconstructed from this projection measurement data, e.g. by way of so-called filtered back projection or using an iterative reconstruction method.
In computer tomography, scattered radiation is a primary cause of image artifacts. The influence of the scattered radiation is proportional to the size of the detection surface of the detector device that is used and/or to the number of x-ray sources. When using quantum-counting energy-resolving detector devices, the scattered radiation can influence the spectral recordings, since the energy of the photons emitted by the x-ray source is changed by the scattering process, e.g. Compton scattering. In general, and in particular in the case of integrating, counting and energy-resolving detector devices, the scattered radiation changes in such a way that scattered photons which have not followed the direct path from the x-ray source to the detector device are detected by the detector device. These detected events result in image artifacts in the reconstruction process, which normally assumes that the photon of the detected event has followed the direct path.
In computer tomography, the influence of the scattered radiation can be reduced at least partially by way of suppression using a scattered ray grating, or by modeling the physical effects and making allowances accordingly. Scattered ray gratings can shield the x-ray detector by absorbing scattered photons from the scattered radiation. The scattered ray grating is situated close to the detector device in this case, i.e. between examination object and detector device, for example. This is disadvantageous in that the detective quantum efficiency (DQE) is reduced and the manufacturing costs for the scattered ray grating are significant.
Focusing and precise alignment of the scattered ray grating with the x-ray source are also necessary. This makes it harder to use a plurality of x-ray sources or a plurality of x-ray focal points with different geometric arrangements, e.g. in computer tomography systems with minimal inverse geometry or in fourth-generation computer tomography systems having a static detector ring and dynamically mobile focal point. Scattered ray gratings are limited in their effect and cannot completely prevent the detection of scattered radiation.
The calculation of scattered radiation can also be performed in conjunction with scattered ray gratings. The calculation of scattered radiation can be effected by directly modeling the physical processes on the basis of an image or the projection measurement data of a recording by the computer tomography system. Calculation methods based on an image are primarily configured to calculate an exact scattered ray distribution, e.g. using Monte Carlo methods, wherein the computing effort is considerable.